1. A thin wispy cloud: Cirrus clouds occur as narrow bands of tiny ice particles, that form at the highest and coldest point of the cloud region.
2. A slender tentacle with sensory or locomotive function, or a part resembling one: An arm of a barnacle can be of a cirrus form.
3. A filament or slender appendage serving as a foot, tentacle,barbel, etc.: Jill read about cirrus, which was a fused group of cilia that functioned like a limb on some protozoans.

A cirrus is a structure formed from a cluster of tuft of fused cilia, constituting one of the sensory or locomotor organs of certain ciliate protozoa.
4. The male copulatory organ of various invertebrates: The flatworm , for example, is described as having a cirrus.
5. A slender flexible animal appendage: ; Cirri are found on barnacles or crinoids,or on many insects and are often tactile.

A cloud species which is unique to the genus cirrus, of such optical thickness as to appear grayish on the side away from the sun, and to veil the sun, conceal its outline, or even hide it completely.

Cirrus clouds are composed of small particles, mostly ice crystals, that are fairly widely dispersed, usually resulting in relative transparency and whiteness and often producing a halo phenomena (colored or whitish rings, arcs, pillars or spots of light) which are not observed with other cloud forms.